The IEEE standard digital communications protocol known as the fibre channel was developed to solve a number of problems known in the art of computer communications. Prior communications protocols, e.g., SCSI (Small Computer Systems Interface) and FICON (Fibre Connection), evolved to require specialized hardware so that multiple protocols could not be supported by the same hardware. Moreover, many prior communications protocols were severely limited in the distances that data could be transported. For example, SCSI is typically limited to 25 meters. The fibre channel protocol addresses both of these problems.
The fibre channel protocol permits data transfers over global distances, and employs a layered strategy providing for the use of other protocols without the requirement for specialized hardware. Particularly, fibre channel acts as a lower layer that views data as standardized packets or frames upon which upper layers, such as SCSI and FICON, may be provided for interpreting the data at the ends of the transmission.
The fibre channel protocol employs two kinds of ports. “N” ports are coupled to peripheral devices with which a computer communicates, and so-called “fabric” ports which are switches within a network of switches that is termed a fabric. There are typically a very large number of fabric and N ports. The N ports are typically arranged in a plurality of distinct loops.
A data packet according to the fibre channel protocol employs three bytes of addressing information. Two of the bytes specify the switches in the fabric to be used to steer the packet to a desired one of the loops. The other byte specifies the address of the N port corresponding to a desired peripheral in the desired loop. Accordingly, another of the advantages of the fibre channel protocol is that a large number of peripheral devices can be addressed.
As can be seen, the fibre channel protocol presumes communication through switches and N ports and, therefore, presumes direct communication with the peripheral devices. A problem arises, therefore, whenever the fibre channel protocol is to be used for communicating with N ports to which no devices are currently connected. This situation occurs, for example, in a library of computer memory elements wherein shelved storage elements must typically be brought to an input/output (“I/O”) station for connection to the port some time after the address identifying the storage element is received.
Accordingly, there is a need for a method and apparatus for emulating a fibre channel port that provides for accessing peripheral devices that are not currently coupled to a port, such as in a library of computer memory elements.